1. Field of the Invention
The present invention relates to phase-locked loop frequency synthesizer devices. More particularly, the present invention relates to a switchable filter and method in a phase-locked loop frequency synthesizer for achieving dual-mode cellular communications.
2. Related Art
A cellular telecommunication system broadly consists of three components: mobile units (such as, portable or car telephones), cellular base stations (cell sites), and mobile telephone switching office (MTSO). Communications between the mobile units and the cell sites are conducted over radio frequency signal channels. Depending on the base station, these channel communications are governed by one of two types of modes or standards, which are commonly referred to as the analog mode (Advanced Mobile Phone Systems (AMPS)) and the digital mode (Digital Advanced Mobile Phone Systems (DAMPS)). Before the advent of the digital communication standard, base stations uniformly used the analog communication standard. However, due to its advantages over analog communication, more base stations are now utilizing digital communication. As a result, it has become necessary for mobile units to have dual-mode capability. Indeed, the North American Digital Cellular Standard (NADC) requires such dual-mode capability. This is known as the Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard (EIA/TIA/IS-54-A, March 1991, published by Electronic Industries Association, Engineering Department, 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006).
This dual-mode requirement, however, places additional burdens on the mobile unit designer. That is, the requirement creates two distinct and conflicting performance specifications for the phase-locked loop (PLL) frequency synthesizer, particularly the loop filter of the synthesizer circuit, employed by these mobile units. The PLL synthesizer is a fundamental component of the mobile unit. The mobile unit includes a transceiver, which comprises a transmitter section and a receiver section. In the transmitter section the PLL synthesizer act as a carrier signal generator and in the receiver section the PLL synthesizer acts as a local oscillator for the communications between the mobile unit and its corresponding cell site. The ability of the PLL synthesizer to eliminate undesired signal interference and undesired noise in both modes has a direct impact on the quality of communications.
The digital mode (DAMPS) effectively requires that the PLL synthesizer utilize a wideband low-pass loop filter (wideband loop filter) to facilitate fast channel switching. This stems from a feature, Mobile Assisted Hand-Off (MAHO), which is unique to the digital mode. Under this requirement, each mobile unit assists the base station in determining which of the available cell channels (frequencies) of the neighboring cell sites is the best to conduct its communications. The mobile unit achieves this by scanning all of the channels that the base station asks it to scan, and identifying the channel with the strongest sustained signal. The measured results of all channels scanned are then reported back to the base station. The base station then processes this information and, if necessary, coordinates the appropriate hand-off (transfer) from one cell site to another.
Accordingly, the PLL synthesizer must be able to switch between channels quickly and efficiently, because the channel scanning has to be done quite often and within a limited period of time. These characteristics correspondingly equate to the need to have a PLL synthesizer with wideband loop filter.
While the use of wideband loop filters allows for a considerable amount of signal phase noise and sideband signals to exist in the synthesized signal, such noise and sidebands do not necessarily affect the quality of communications. This is because digital communications inherently have a high signal-to-noise immunity ratio which allows for the toleration of signals with low spectral purity. In other words, digital communications allow for the PLL synthesizer to have a relaxed loop filer (that is, wideband).
In contrast, it is desirable in the analog mode (AMPS) to have a narrowband low-pass filter (narrowband filter) to eliminate signal phase noise and sidebands. In analog communications, phase noise has a direct effect on the quality of communications. Thus, a narrowband filter is desired to maintain a higher spectral purity of the synthesized signals. The penalty with the use of narrowband filters, however, is that they cause the PLL synthesizer to have slow switching times and less loop stability.
Conventional systems have addressed this inherent conflict generally in two ways. Some have simply ignored the low spectral purity problem in the analog mode, and have employed an optimized wideband loop filter for the digital mode in order to satisfy the fast channel switching time requirement of MAHO. While relatively uncomplicated and cost efficient, the disadvantages of this type of system is the degradation of the communications quality and selectivity in the analog mode. That is, the signal noise not removed by the wideband filter is perceivable by the user whereas the sidebands degrade the mobile unit's ability to reject unwanted interferences.
Other conventional systems provide dual-mode cellular communication with a fractional N synthesizer, which provide for better spectral purity in the analog mode, concurrent with the fast channel switching capability. The drawbacks of these types of systems, however, are their considerable complexity and cost.